Over the past decade a pursuit of solid state ultrafast scaleable devices based on both the charge and spin of an electron has led to a development of new fields of magnetoelectronics and spintronics. The discovery of giant magnetoresistance (GMR) in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phenomenon where a relatively small change in magnetism results in a large change in the resistance of the material.
The phenomenon of a large tunnel magnetoresistance (“TMR”) of ferromagnet-insulator-ferromagnet (“F1-I-F2”) structures is a focus of product development teams in many leading companies. TMR is typically observed in F1-I-F2 structures made of two ferromagnetic layers, F1 and F2, of similar or different materials separated by the insulating thin tunnel barrier I with thickness typically ranging between 1.4–2 nm.
It is worth mentioning recent studies of the giant ballistic magnetoresistance of Ni nanocontacts. Ballistic magnetoresistance is observed in Ni and some other nanowires where the typical cross-section is a few square nanometers. The transport in this case is through very short constriction made on the vicinity of the nanowire and it is thought to proceed with conservation of electron momentum (ballistic transport). The change in the contact resistance can exceed 10 fold (or over 1000%).
Of particular interest has been the injection of spin-polarized carriers, mainly in the form of spin-polarized current into semiconductors. This is significant due to relatively large spin-coherence lifetime of electrons in semiconductors, including possibilities for use in hetero laser and light-emitting diodes of polarized radiation. Development of sources of stimulated and spontaneous polarized radiation, i.e., laser and a light-emitting diode of polarized light is one of the most urgent problems of optical communication. Conventional sources have low degree of polarization.
FIG. 1A illustrates a schematic prototypical model of a conventional double hetero laser and light-emitting diode 100. As shown, the diode 100 includes a first semiconductor layer 110, a second semiconductor layer 120 below the first semiconductor layer 110, and a third semiconductor layer 130 below the second semiconductor layer 120. The diode 100 also includes a substrate 140 below the third semiconductor layer 130 and first and second contacts 150 and 160 above the first semiconductor layer 110 and below the substrate 140, respectively.
The first semiconductor layer 110 is relatively heavily negatively doped (n+) and the third semiconductor layer 130 is relatively heavily positively doped (p+). The second semiconductor layer 120 may be either positively (p) or negatively (n) doped, but as a rule, the dopant concentration level is less than that of the first or the third semiconductor layers 110 or 130. Main feature of the double heterostructure is that the second semiconductor layer 120 has the narrower band gap when compared to the band gaps of the adjacent first and second semiconductor layers 110 and 130.
FIG. 1B illustrates an energy band diagram of the diode 100 illustrated in FIG. 1A along the line I—I at equilibrium. In this figure, the Fermi level EF, the bottom conduction band energy level EC, and the top valence band energy level EV are shown. Also, the energy band gaps for each material Eg1 (first semiconductor layer 110), Eg2 (second semiconductor layer 120), and Eg3 (third semiconductor layer 130) are shown where Egi=ECi−EVi for each layer, i=1–3. As mentioned above, Eg2<Eg1, Eg3.
FIG. 1C shows the same as FIG. 1B, but at a large bias. Radiation in the diode 100 is generated as a result of radiative recombination of non-equilibrium electrons and holes in the second semiconductor layer 120. The electrons and holes are injected into the second semiconductor layer 120 (which has narrower band gap) from first and third semiconductor layers 110 and 130, respectively. The electrons and holes are only slightly spin-polarized (due to weak spin-orbital coupling) in the conventional light-emitting heterostructure such as the diode 100. Consequently, the radiation has a very low degree of polarization.
The possibility of spin injection from ferromagnetic semiconductors (FMS) into nonmagnetic semiconductors has been demonstrated in a number of recent publications. However, the Curie temperature (the temperature above which a material becomes nonmagnetic) of magnetic semiconductors is substantially below room temperature. The low Curie temperature limits possible applications. Room-temperature spin injection from ferromagnets (FM) into semiconductors has also been demonstrated, but its efficiency is very low (˜1–2%).
The main problem of the spin injection from a ferromagnet into semiconductor is that a potential barrier (Schottky barrier with the height Δ) for carriers always forms in the semiconductor near the metal-semiconductor interface due to different values of the electrode work function and the affinity of a semiconductor. Numerous experiments have shown that the barrier height Δ is determined by surface states forming at the interface, which is approximately (⅔)Eg almost independent of the type of a metal, where Eg is the energy band gap of the semiconductor. For example, in GaAs Eg≈1.42 eV and Δ≈0.8–1.0 eV, in the case of Si Eg=1.12 eV and Δ≈0.6–0.8 eV.